In the past sulfurized sperm oil has been used as an additive (usually as a friction modifier) in many lubricant formulations, such as in gear oils, worm and spur gears, automatic transmission fluids, wax lubricants, Permawick lubes for sintered bronze or sintered babbit bearings, and as a cutting oil additive. Sulfurized sperm oil has now become quite expensive (about $.40 per pound) and the present invention is directed to a relatively inexpensive (about $.20 per pound) replacement for sulfurized sperm oil. Sulfurized olefins alone do not have the lubricity necessary for many applications, as is indicated by their high slip/stick ratios. Sulfurized lard oil is not adequately soluble in paraffinic base oils. It has now been found that by sulfurizing a blend of lard oil and polyisobutylene together, a material (or "co-reaction product") is obtained which has good lubricity and is soluble in paraffinic base lube stocks.